Electro-optical display device using a liquid crystal cell

ABSTRACT

An electro-optical display device and a method of manufacturing it are disclosed. The display device includes a liquid crystal cell arranged between two polarizers. A light source is positioned adjacent one polarizer on the side thereof remote from the liquid crystal cell, and an electronic control device is coupled to the liquid crystal cell. A diaphragm is arranged on one side of the liquid crystal cell in the form of an aperture for limiting the display field of the liquid crystal. The device limits viewing parallax and improves contrast for both day and night viewing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns an electro-optical display device, consisting ofa liquid crystal cell (LC cell) arranged between two polarizers, a lightsource, arranged on the side of one polarizer remote from the LC cell,and an electronic control device. The invention further concerns aprocess for the manufacture of such display devices. 2. Description ofthe Prior Art

Display devices of the kind above-mentioned are known. In daylight theyare operated as reflection cells, the ambient light being employed asthe light source. In darkness, on the other hand, these devices operateas transmission cells. In this case a light source arranged behind thesecond polarizer is employed for illumination.

In a few known devices, for local selective illumination of the signs orsymbols to be displayed, a diaphragm is disposed between these symbolsand the light source. In order to keep as small as possible the readingerror (parallax), which results due to the distance between the localillumination device and the liquid crystal layer, the width of theaperture in the diaphragm must be chosen appropriately small.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a display devicewhich makes possible a reading of displayed symbols with littleparallax.

A further object of the invention is to provide dark symbols on a brightbackground by day and bright symbols on a dark background by night,whereby the energy required for internal illumination will be reduced toa minimum, and physiologically favorable reading conditions will beprovided.

Yet another object of the invention is the provision of a method ofproducing an improved display device of the type described above.

Briefly, these and other objects are achieved in accordance with thepresent invention in that for local selective illumination of the rangeof symbols to be displayed, a diaphragm is arranged on the side of oneof the two plates limiting the liquid crystal turned towards the liquid,and in that the form of the aperture of this diaphragm corresponds tothe form of the respective symbol field.

In order to ensure good contrast between the displayed symbols, andtheir surroundings, the regions of the diaphragm which are opaque to theinternal illumination must, when the display device is operating byreflection, exhibit about the same brightness as the remainingbackground of the symbols.

The method of manufacturing of the display device in accordance with theinvention is characterized in that after the application of theelectrode pattern, the opaque regions of the diaphragm are provided withan electrically insulating masking layer.

As compared with the application of the masking layer defining thediaphragm before the production of the electrode pattern, this has theadvantage that the production of the conductive layer can be effected ona unitary substrate, and the etching of the electrode structure can beeffected without taking heed of the characteristics of the diaphragmlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a sectional view through a display device with an electroniccontrol device associated therewith;

FIG. 2 is a plan view of a light distributor preferably employed as thelight source for a display device with several symbol fields to bedisplayed;

FIGS. 3a to 3c are timing diagrams showing the time relations in theelectronic control device of FIG. 1 for day and night operation; and

FIG. 4 is a further electronic control device for use with the displaydevice of FIG. 1

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the serveral views, and moreparticularly to FIG. 1 thereof, there is shown a display device 1, thatcontains an LC cell 2, which consists of two glass plates 3 and 4 and aliquid crystal 5 arranged between these glass plates. As to the liquidcrystal 5 this may be, for example, a nematic compound which possessesthe capability of optical rotation, that is electrically controllablewith the help of two electrodes 6 and 7 placed on the inner surfaces ofthe glass plates 3 and 4.

The electrodes 6 and 7 are transparent and consist of an electricallyconductive material (for example, tin oxide), that is evaporated orotherwise applied upon the glass plates 3 and 4. They correspond intheir form to the symbols which are to be displayed. The electrodes 6and 7 are connected via respective electrical leads 8 and 9 with acontrol device 10, that in turn consists of a control stage 11 and anEXCLUSIVE-OR gate 12.

On the outer surfaces of each of the glass plates 3 and 4 are arrangedpolarizing foils 13 and 14 respectively. Abutting the polarizing foil 14is arranged a semi-transparent reflector 15, to the outer surface ofwhich there is attached a light distributor 16.

The regions of the inner surface of the glass plate 3 abutting theelectrode 6 are provided with a mask opaque to the internalillumination, which therefore represents a diaphragm 17, of which theapertures exhibit the form of the symbols to be displayed. The maskconsists, for example, of magnesium oxide or another substance that muston the one hand be electrically non-conducting and on the other handexhibits a reflectivity of about the same brightness as the remainingbackground.

If, as shown in FIG. 1, the mask is applied to the inside of the upperglass plate 3, then in the covered regions of the display layer thesurface orientation of the liquid crystal compound 5 has no importance.Only the symbol regions defined by the form of the electrode 6 have afunction. Thus defects in orientation of the display background areabsent as a reason for rejection. This advantage is also attainedtherethrough that, instead of the masking layer 17, a diaphragm isarranged between the polarizer 13 and the glass plate 3. In this case,however, reading errors appear once more owing to the parallax that isthen present.

The light distributor 16 consists, for example, of a plexiglass plate,in a bore 18 of which is inserted an incandescent lamp (orlight-emitting diode) 19. Since typical miniature incandescent lampshave a diameter of 1.5 mm, the incandescent lamp 19 is preferablyarranged laterally of the LC display, so as not to make the overallthickness of the display and illumination too great (which isparticularly important in the application of such display devices forwrist watches). The surfaces of the light distributor 16 turned awayfrom the polarizer 14 are mirrored and serve as light reflectors.

In order to allow as much light as possible to appear in darkness(transmission operation) through the transparent electrode 6 and thusthrough the symbol region to be illuminated, the otherwise smoothplexiglass surface is roughened or provided with notches in the region20 opposed to the electrode 6. The width D of the region 20 shouldcorrespond at most to the width d of the electrode 16. Since in thismanner the light emerges from the light distributor only in the segmentregions, a higher light intensity can be attained than with whole-areaillumination.

In FIG. 2 is represented a plan view of another embodiment of a lightdistributor of a type which displays several symbol regions. The lightemissive surfaces 20^(I), . . . 20^(VII) are connected by individuallight-guides 16^(I), . . . 16^(VII) with the schematically indicatedincandescent lamp 19^(I). The light-guides may be fibre-optic guides orinjection moulded plastic elements. The ends of the light guides 16^(I),. . . 16^(VII) are here identical with the light-emissive areas 20^(I),. . . 20^(VII), which lie in a common plane below the LC display.

The advantage of the light distributor in accordance with FIGS. 1 and 2as compared with the known devices (black diaphragm and uniformillumination) consists especially in the higher light intensity whichcan be attained in the local limited areas.

As the reflector 15, the reflector disclosed in Swiss patent applicationNo. 8375/75 and corresponding U.S. application Ser. No. 689,251 isparticularly suitable, which possesses a light scattering surface on theside turned towards the polarizer 14, that consists of a highlyreflective thin metal layer (some 200 A), which is applied to a layersupport provided with grooves. The semi-transparent mirror 15 can alsobe omitted. In this case the mirrored rear surface of the lightdistributor 16 serves as a reflector for daylight.

The mode of operation of the control device 10 is seen from the FIGS. 3ato 3c:

FIG. 3a shows the square-wave variation of voltage generated by thecontrol stage 11 (for a clock display device this means a time-basecircuit with following decoding and driver circuits), which appears atthe input A of the EXCLUSIVE-OR gate 12 (FIG. 1). If a logical `one`exists at the input B of this gate 12 (condition 1), then there resultsat the output C of the gate 12, and thus on the electrode 7, a pulsetrain (FIG. 3c) which exhibits a phase-shift of 180° as compared withthe pulse train at the input A of the EXCLUSIVE-OR gate 12.

Provided that the voltage variation on the electrode 6 corresponds withthat of FIG. 3a and provided the polarizers 13 and 14 are mutuallycrossed, the region of the symbol given by the form of the electrode 6becomes dark when a voltage variation in accordance with FIG. 3c appearson the electrode 7.

If on the contrary a logical zero (condition 2) is present at input B ofthe EXCLUSIVE-OR gate 12, then there results at output C of the gate 12,and so on the electrode 7, a pulse train (FIG. 3b) which shows no phaseshift with respect to the pulse train at input A of the EXCLUSIVE-ORgate 12.

Accordingly, in condition 1 dark symbols appear on a bright background(reflection operation in daytime). In darkness, on the contrary,condition 2 is provided, i.e. bright symbols on a dark background. Thetransition from condition 1 to condition 2 is brought about either by apush button or or by a photocell signal.

For wrist watch displays with incandescent lamps or light-emissivediodes as internal light sources a control button is built in because ofthe high power requirement. Here the control signal at input B of theEXCLUSIVE-OR gate may be derived from the same button.

If on the other hand there are used as light sources phosphor layersexcited by radiation (uninterrupted illumination), then a photodiode isprovided. If the ambient light differs from a definite value, thencomplementary control is effected.

In FIG. 4 there is represented a transmission gate circuit 12^(I), whichis likewise suitable for phase control of the volatage on electrode 6.The two inputs are again designated by A and B and the output by C. Inaccordance with the control signal at input B (or 1) either the gate 21or the gate 22 is opened. The elements 23 and 24 are logical inverterstages. The mode of operation of this transmission gate circuit 12^(I)is the same as explained above with reference to FIGS. 3a - 3c.

Obviously, numerous additional modifications and variations of thepresent invention are possible in light of the above teachings. It istherefore to be understood that within the scope of the appended claims,the invention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States:
 1. An electro-optical display device, comprising:aliquid crystal cell including two transparent plates bounding a liquidcrystal layer and arranged between first and second polarizers; a lightsource arranged on the side of one of the polarizers remote from saidliquid crystal cell; an electronic control device, coupled to saidliquid crystal cell; and, a diaphragm positioned on the side of one ofthe two plates of said liquid crystal cell which is adjacent to theliquid crystal layer, said diaphragm including an opening correspondingin form with the form of the desired symbol field of said liquid crystalcell for producing selective local illumination of said symbol field. 2.An electro-optical display device in accordance with claim 1wherein:said light source consists of a light distributor coupled withat least one incandescent lamp.
 3. An electro-optical display device inaccordance with claim 1 wherein:said light source consists of a lightdistributor coupled with at least one light-emissive diode.
 4. Anelectro-optical display device in accordance with claim 2 wherein:saidlight distributor consists of a transparent material havings its faceroughened in the regions adjacent to which said symbol fields to beilluminated are situated.
 5. An electro-optical display device inaccordance with claim 4 wherein:the width of said roughened region is nogreater than the width of the corresponding display field.
 6. Anelectro-optical display device in accordance with claim 4 furthercomprising:a light reflective coating provided on the surface of saidlight distributor remote from said first polarizer.
 7. Anelectro-optical display device in accordance with claim 2 furthercomprising:a semi-transparent mirror positioned between said firstpolarizer and said light distributor.
 8. An electro-optical displaydevice in accordance with claim 7 further comprising:a plurality ofsymbol fields to be displayed; and, light guides having respective lightexit surfaces of the same form as each of said symbol fields to bedisplayed arranged with all said light exit surfaces in a common planeat the rear of said semi-transparent mirror.
 9. An electro-opticaldisplay device in accordance with claim 7 wherein:said semi-transparentmirror includes on the side turned towards said first polarizer a lightscattering surface that consists of a highly reflective thin metal layerwhich is applied to a supporting member provided with grooves.
 10. Anelectro-optical display device in accordance with claim 9 wherein:saidsupporting member is formed of glass and said reflective metal layerconsists of a silver layer between 100-400 A thick.
 11. Anelectro-optical display device in accordance with claim 1, wherein saidelectronic control device comprises:an EXCLUSIVE-OR gate; and means forsupplying a pulse train and a logic control signal to said EXCLUSIVE-ORgate.
 12. An electro-optical display device in accordance with claim 1,wherein said electronic control device comprises:a pair of AND gates;means for supplying a pulse train and the inverse of a control signal toone of said pair of AND gates; and means for supplying the controlsignal and the inverse of the pulse train to the other of said pair ofAND gates.
 13. A method of manufacturing an electro-optical displaydevice including a liquid crystal cell arranged between two polarizersand having a field limiting diaphragm on said liquid crystal cell,comprising the steps of:applying an electrode pattern to said liquidcrystal cell; and subsequently, providing non-translucent regions ofsaid diaphragm with an electrically insulating masking layer.